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<br />flow that is not seriously damaging. This is because those who are <br />damaged soon learn to avoid frequent damage if it is serious. It <br />should be noted, however, that reservoir control often reduces the <br />frequency of such flows, and the tendency to use flood-prone areas un- <br />wisely must be controlled by regulations, particularly when the flood- <br />control effectiveness of reservoirs depends on the availability of <br />downstream channel capacity. <br />In a planning study in which the storage and release capacity and <br />operation rules for a flood control reservoir are being determined, <br />allowance must be made for imperfection in operation as discussed in <br />Volume 1. Experience in the western United States has shown that for <br />design purposes, the target flow at damage locations should be about <br />80 percent of the actual flow above which significant damages occur. <br />Then, in actual operation, every effort should be made to utilize <br />effectively all of the flow capacity. If this is done, the effective- <br />ness of actual operation can reasonably approach the design objectives. <br />Where a reservoir is operated to regulate floods at locations a <br />considerable distance downstream, allowance must be made for local <br />runoff that will occur downstream of the dam and above the damage <br />area. The release from the reservoir is determined as the difference <br />between the target flow and the maximum local runoff forecasted to <br />occur during the times when a portion of the current releases will <br />reach the damage area. Maximum forecasted local runoff in the applica- <br />tion is the "best forecast" amount plus a contingency allowance. This <br />contingency allowance usually ranges from 25 to 100 percent of the <br />local runoff during rain floods, because forecast accuracy for rapidly <br /> <br />2-02 <br />